In the past, a magnetic resonance imaging apparatus for generating static magnetic field using a superconductive magnetic device has been widely used particularly in the field of clinical diagnosis. A magnetic resonance imaging apparatus is capable of acquiring images which exhibit physical and chemical properties of an object to be examined, using nuclear magnetic resonance phenomenon generated when high-frequency pulses are irradiated to the object (examinee) placed in a homogeneous static magnetic field space. A magnetic resonance imaging apparatus is configured mainly comprising a magnetic device as a magnetic field generating source for applying homogeneous static magnetic field in an imaging region into which the object is to be carried, an RF coil for irradiating high frequency pulses toward the imaging region, a reception coil for receiving response from the imaging region and a gradient magnetic field coil for applying a gradient magnetic field so as to provide positional information of resonance phenomena to the imaging region.
In magnetic resonance imaging apparatus, one of the requirements for improving image quality is improvement of static magnetic field homogeneity. Given this factor, upon designing and manufacturing of a magnetic device to be used for a magnetic resonance imaging apparatus, adjustment of magnetic field is carried out in the respective steps of designing, assembling and installation, in order to make uniform the static magnetic field generated in the imaging region by a magnetic field generating source.
As for the magnetic field adjustment in the installation step for the purpose of adjusting inhomogeneous components generated due to manufacturing errors or surrounding environment, it is often carried out by incrementally disposing or taking off a magnetic homogeneity adjusting body formed by a magnetic material to a magnetic device (for example, refer to Patent Document 1). This magnetic field homogeneity adjusting body is generally disposed in the space between the magnetic field generating source and a gradient magnetic field coil to be disposed inside thereof (imaging region side) using a magnetic field homogeneity adjusting mechanism (means), etc.
On the other hand, magnetic field homogeneity of a magnetic device varies on a long-term or short term basis due to environmental fluctuation such as air temperature. Here, even though it is possible to adjust magnetic field using the above-mentioned magnetic field homogeneity adjusting body in order to further correct fluctuation of magnetic field homogeneity, it is not appropriate to perform such adjustment of magnetic field homogeneity every time since it usually takes a long time for such adjustment operation. Given this factor, various methods have been developed for absorbing such fluctuation of magnetic field homogeneity.
One of them is the method to mount in advance a plurality of coils for adjusting a magnetic field in the magnetic device and/or a gradient magnetic field coil, calculate the current value for compensating fluctuation of magnetic field homogeneity, and energize the relevant coil. For example, in Patent Document 2, the technique is disclosed to measure the temperature of a magnetic material which configures the magnetic circuit of the magnetic device, calculate the applied current value of the coil for adjusting the magnetic field based on the measured temperature and distribute power to the coil based on the relevant applied current value, so as to obtain a homogeneous magnetic field.
On the contrary, the method for suppressing temperature fluctuation of the magnetic material itself has also been developed. For example, in Patent Document 3, the method is described for covering the static magnetic field generating source with heat insulating material, heating material and a heater so as to keep a constant temperature of the static magnetic field generating source by applying heat with the heater. Also in Patent Document 4, the method is described for disposing a cooling device in addition to heating so as to improve accuracy of temperature control by freely using the provided devices.
Further, the method has been developed for achieving magnetic field homogeneity by actively using distribution of magnetization intensity generated by temperature distribution of the magnetic material. For example, the method is described in Patent Document 5 for obtaining magnetic field homogeneity by adjusting temperature distribution of a permanent magnet, and the method for obtaining magnetic field homogeneity by adjusting temperature distribution of the entire magnetic device is described in Patent Document 6. Also, while the superconductive magnet comprising a pole piece is described in Patent Document 7, adjustment of temperature distribution of the pole piece is not described therein.
Patent Document 1: Japanese Patent No. 3733441
Patent Document 2: Japanese Patent No. 3781166
Patent Document 3: JP-B-H3-28931
Patent Document 4: JP-A-H3-109043
Patent Document 5: JP-A-2003-116807
Patent Document 6: Japanese Patent No. 3559364
Patent Document 7: Japanese Patent No. 3559364